It is hypothesized that higher indoor nitrogen dioxide levels cause diarrhoea in infants and that this is the result of a direct action of oxides of nitrogen on the gut. This hypothesis is tested by reviewing the reported association between methaemoglobin and diarrhoea in children and two recent reports on indoor air and diarrhoea in infants. The collection of further empirical data is now needed. Studies which measure indoor levels of nitrogen dioxide could usefully collect data on infants symptoms that are not exclusively respiratory. Similarly, studies which are collecting diary information on children's health symptoms should consider collecting data on indoor air quality with respect to the oxides of nitrogen.